Evolution of interlayer and intralayer magnetism in three atomically thin chromium trihalides

Significance Two-dimensional magnetic semiconductors such as CrI3 are a new class of van der Waals material that may allow for the development of novel 2D spintronic devices. While strong magnetic anisotropy within the CrI3 layers stabilizes ferromagnetism down to a monolayer, weak antiferromagnetic coupling between the layers gives rise to extremely large tunnel magnetoresistance. We use a combination of tunneling and magneto-optical measurements to investigate the entire 2D chromium trihalide family (CrX3, X = I, Br, Cl). Our results elucidate both the interlayer coupling and intralayer spin Hamiltonian for all three materials, and further demonstrate that ferromagnetism can be stabilized in monolayer CrBr3 and bilayer CrCl3 without strong anisotropy. We conduct a comprehensive study of three different magnetic semiconductors, CrI3, CrBr3, and CrCl3, by incorporating both few-layer and bilayer samples in van der Waals tunnel junctions. We find that the interlayer magnetic ordering, exchange gap, magnetic anisotropy, and magnon excitations evolve systematically with changing halogen atom. By fitting to a spin wave theory that accounts for nearest-neighbor exchange interactions, we are able to further determine a simple spin Hamiltonian describing all three systems. These results extend the 2D magnetism platform to Ising, Heisenberg, and XY spin classes in a single material family. Using magneto-optical measurements, we additionally demonstrate that ferromagnetism can be stabilized down to monolayer in more isotropic CrBr3, with transition temperature still close to that of the bulk.